lxc: Move lxcContainerAvailable to virprocess

lxc: Move lxcContainerAvailable to virprocess

Other drivers (like qemu) would like to know if the namespaces
are available therefore it makes sense to move this function to
a shared module.

At the same time, this function had some default namespaces that
are checked with every call. It is not necessary - let callers
pass just those namespaces they are interested in.

With the move the function is renamed to
virProcessNamespaceAvailable.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>